Materials Transactions, Vol. 49, No. 3 (2008) pp. 506 to 512
#2008 The Japan Institute of Metals
Excellent Thermal Stability and Bulk Glass Forming Ability
of Fe-B-Nb-Y Soft Magnetic Metallic Glass
Sangmin Lee1; *1 ,*2 , Hidemi Kato2 , Takeshi Kubota2 , Kunio Yubuta2 ,
Akihiro Makino2 and Akihisa Inoue3
1
Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
3
Tohoku University, Sendai 980-8577, Japan
2
Effects of Y addition on the glass forming ability (GFA) of a Fe-B-Nb marginal glass former, and annealing effects on glassy/supercooled
liquid phases as well as soft magnetic properties in the multicomponent Fe-B-Nb-Y alloy system were investigated. The origin of highly
improved GFA in the multicomponent system is discussed with related to a characteristic exothermic phase transformation, chemical short range
ordering, in the supercooled liquid region due to the positive heat of mixing between Nb-Y elements. The separating tendency between Nb and Y
elements is considered to suppress precipitation of metastable Fe23 B6 and -Fe crystalline phases, thus to result in highly improving GFA and
distinct high thermal stability against heat treatment of the alloy system. In addition, a glassy ring was fabricated by copper mold casting and
magnetic properties were investigated before/after heat treatment. [doi:10.2320/matertrans.MBW200732]
(Received October 30, 2007; Accepted January 11, 2008; Published February 20, 2008)
Keywords: metallic glasses, iron-based alloy, high thermal stability, excellent soft magnetic properties
1.
Introduction
More and more attention has been paid to Fe-based bulk
metallic glasses (BMGs) due to the high fracture strength,
hardness, corrosion resistance1–3) and their excellent softmagnetic properties. The excellent soft-magnetic Fe-based
BMGs are considered to be useful for magnetic field
shielding materials, transformer core materials and perpendicular magnetic memory device. A soft-magnetic underlayer, absolutely necessary for the perpendicular magnetic
memory device, is required to have high thermal stability
enough to sustain the good soft-magnetism during thermal
treatments. What is more, the Fe-based BMG is one of the
most appropriate candidates for precision devices such for
micro-motor and NEMS associated with the nanoimprinting
technique, since these have good merits for processing of
extremely fine configuration by the Newtonian viscous flow
at the supercooled liquid region as well as good soft-magnetic
properties, comparing with both conventional crystalline
materials and non-ferrous BMGs. In spite of these advantages, low glass forming ability (GFA) and poor thermal
stability of the Fe-based BMGs have restricted those
industrial applications. Recently, the effects of the rare earth
(RE) elements such as Y upon the Fe-based glass formers are
reported by DH. Kim et al. (Yonsei Univ., Korea).4) A lot of
scientists in the world are investigating the effectiveness of
the rare earth elements addition upon the Fe-based glass
formers, and a couple of reports about the high GFA of the Y
added Fe-based amorphous alloys have been already published. Nevertheless, the magnetic properties of the Y bearing
Fe-based BMGs are not included in the almost reports.4–11)
In this paper, the effects of Y addition on GFA of a Fe-BNb marginal glass former, and annealing effects on glassy/
supercooled liquid phases and soft magnetic properties in the
*1Corresponding
*2Graduate
author, E-mail: smlee@imr.tohoku.ac.jp
Student, Tohoku University
multicomponent Fe-Nb-B-Y alloy system were investigated.
The origin of highly improved GFA in the multicomponent
system is discussed with related to a characteristic exothermic phase transformation, chemical short range ordering, in
the supercooled liquid region due to the positive heat of
mixing between Nb-Y elements. The separating tendency
between Nb-Y is considered to suppress precipitation of
metastable Fe23 B6 and -Fe crystalline phases, thus to result
in highly improving GFA and distinct high thermal stability
against heat treatment of the alloy system. This tendency is
also considered to raise the internal stress frozen in the ascast BMG, which degrades the soft magnetic properties, i.e.
coercivity and permeability. However, these soft magnetic
properties could be improved by the appropriate heat
treatments enabled by the distinct high-thermal stability of
this alloy composition.
2.
Experimental Procedures
Glassy ribbons and rods of (Fe0:72 B0:24 Nb0:04 )100x Yx
(x ¼ 0 6) were prepared in an argon atmosphere by the
single-roller melt spinning technique and copper-mold casting technique, respectively. Firstly, Fe72 B24 Nb4 pre-alloy
ingots were obtained by induction melting with Fe (99.9%),
B (99.5%), and Nb (99.9%), and then RE element, Y (99.9%)
were added to the pre-alloy ingot by the arc-melting method.
And then, the surface oxide layer on the ingot was removed
with a grinder in order to prevent a heterogeneous nucleation.
The thermal stability and phase transition behavior of
alloys were observed by a differential scanning calorimeter
(DSC) at a heating rate of 0.67 K/s. The supercooled liquid
temperature region, which is defined as a temperature spun
between the glass transition temperature (Tg ) and the onset
temperature of crystallization (Tx ), Tx ¼ Tx Tg was also
calculated. In addition, the melting temperature (Tm ) and
liquidus temperature (Tl ) were measured at a cooling rate of
0.17 K/s after complete melting.
Excellent Thermal Stability and Bulk Glass Forming Ability of Fe-B-Nb-Y Soft Magnetic Metallic Glass
Fe96-xBxNb4
Heating Rate = 0.67 K/s
(x= 16, 20, 24, 28 and 32)
Exothermic
x=16
Tg
x=20
x=24
x=28
x=32
Tx
500
600
700
800
900
1000
1100
Temperature/K
Fig. 1 DSC traces obtained from Fe96x Bx Nb4 (x ¼ 16, 20, 24, 28 and 32)
marginal metallic glasses at a heating rate of 0.67 K/s.
The glassy specimens were annealed at different temperatures depending on their Tg and Tx at a heating rate of
0.67 K/s for various annealing periods to investigate their
phase transition and crystallization behaviors. The glassy and
crystalline phases were confirmed by a X-ray diffractometer
(XRD, Rigaku, RAD-B) with Cu-K radiation from 20 to 80
degree of 2. Nanocrystallization and other phase transition
behaviors were observed by using a transmission electron
microscope (TEM, JEOL).
Magnetic properties were measured with a vibrating
sample magnetometer (VSM, Digital Measurement Systems,
VSM-5) for saturation magnetization, B-H loop Tracer for
coercivity, respectively. A glassy ring and an amorphous
ribbon of (Fe0:72 B0:24 Nb0:04 )95:5 Y4:5 were used for coercivity
measurement for comparison.
3.
Results and Discussions
3.1
Glass forming ability of (Fe0:72 B0:24 Nb0:04 )100 x Yx
alloy system
Fe96x Bx Nb4 (x ¼ 16, 20, 24, 28 and 32) compositions
were studied for searching the best pre-alloy. The glass
transition phenomenon was observed at x ¼ 20, 24 and 28,
and the largest value of Tx (¼ 39 K) was obtained at x ¼ 24
(Fe72 B24 Nb4 ) as shown in Fig. 1. The ternary Fe-B-Nb
system has been already studied by Imafuku et al.12) They,
507
however, investigated the crystallization behavior of
Fe90x Bx Nb10 (x ¼ 10, 20 and 30) glassy ribbons and
discussed the relationship between the stability of supercooled liquid state and the phases formed upon crystallization
from the structural point of view. In this study, Nb content is
limited to 4 at%, as considering of the cost and magnetic
properties. Furthermore, the high GFA and good magnetic
properties are reported in the recent Fe-based BMGs of 4 at%
of Nb bearing compositions. Therefore, Fe72 B24 Nb4 was
selected as a base alloy in this study.
According to the empirical rules suggested by Inoue
(Tohoku Univ., Japan),13) Y is good candidate for improving
GFA of the base alloy since the atomic size difference is 12%
or more with Fe, B and Nb. In addition, the heats of mixing
among Y and the constituent elements become also large
negative values, except for Nb-Y. From thermal properties
Tg , Tx , Tm and Tl , Tx , the reduced glass transition temperature Trg ¼ Tg =Tl and ¼ Tx =ðTg þ Tl Þ, were calculated
for estimating GFA of the alloys in Table 1. As increasing Y
addition in (Fe0:72 B0:24 Nb0:04 )100x Yx alloy system, Tg , Tx
and Tx became larger and the value of Tx at 6 at% Y
addition mounts up to 125 K which is considered to be the
largest supercooled liquid region among the Fe-based
amorphous or glassy alloys developed up to date. But Trg
and were the highest at 4.5 at% Y addition since the factors
of Trg and are relying on the Tl . The critical glass forming
size was enhanced from 0.02 mm in thickness for the base
alloy ribbon up to 7 mm in diameter for the 4.5% Y added
alloy. Both Trg (¼ 0:645) and (¼ 0:442) are the highest at
x ¼ 4:5%. Therefore, these parameters are found to catch the
trend of GFA exactly in this alloy system.
3.2
Characteristic exothermic reaction within the supercooled liquid region in (Fe0:72 B0:24 Nb0:04 )100 x Yx
alloy system
The mixing enthalpy of Nb-Y is +30 kJ/mol. It means
that these elements have a tendency to separate each
other during the solidification process.14–18) Even though
(Fe0:72 B0:24 Nb0:04 )100x Yx (x ¼ 0 10) alloys have the large
positive mixing enthalpy between Nb-Y, they could be frozen
into a meta-stable homogeneous glassy phase due to the high
cooling rate. As shown in Fig. 2, it is worthy of pointing out
that an exothermic reaction is found within the Tx of the
alloys with 3:5 at% Y element.
Figure 2 shows DSC traces obtained from the as-quenched
Fe72 B24 Nb4 glassy ribbon, as-cast (Fe0:72 B0:24 Nb0:04 )95:5 Y4:5
Table 1 Thermal properties and maximum glass forming diameter of (Fe0:72 B0:24 Nb0:04 )100x Yx (x ¼ 0, 2, 3, 4, 4.5, 5, and 6) metallic
glasses, where Tg : glass transition temperature, Tx : crystallization temperature, Tx : temperature spun of the supercooled liquid state
(¼ Tx Tg ), Tm : melting temperature, Tl : liquidus temperature, reduced glass transition temperature (¼ Tg =Tl ), parameter for the glass
forming ability (¼ Tx =ðTg þ Tl Þ) and Dmax : the maximum glass forming parameter by a copper mold casting technique.
Composition
Tg (K)
Tx (K)
Tx (K)
Tm (K)
Tl (K)
Trg
Dmax (mm)
Fe72 B24 Nb4 (base alloy)
837
876
39
1359
1497
0.559
0.375
0.02 (ribbon)
(Fe0:72 B0:24 Nb0:04 )98 Y2
855
918
63
1325
1473
0.580
0.394
2.0
(Fe0:72 B0:24 Nb0:04 )97 Y3
860
942
82
1326
1430
0.601
0.411
4.0
(Fe0:72 B0:24 Nb0:04 )96 Y4
(Fe0:72 B0:24 Nb0:04 )95:5 Y4:5
861
871
959
982
98
111
1326
1326
1349
1349
0.638
0.645
0.434
0.442
6.0
7.0
(Fe0:72 B0:24 Nb0:04 )95 Y5
870
990
120
1324
1445
0.602
0.427
6.0
(Fe0:72 B0:24 Nb0:04 )94 Y6
883
1008
125
1330
1431
0.617
0.436
5.0
508
S. Lee et al.
Heating Rate = 0.67 K/s
Tg
(a)
Exothermic
Tx
(b)
Tx
(c)
Tx
500
600
700
800
900
11000
1100
Temperature/K
Fig. 2 DSC traces obtained from (a) the as-quenched Fe72 B24 Nb4 glassy
ribbon, (b) as-cast (Fe0:72 B0:24 Nb0:04 )95:5 Y4:5 BMG (3.0 mm in diameter),
and (c) the specimen preliminary annealed up to 928 K. The characteristic
exothermic reaction observed in (b) was disappeared by the preliminary
heating up to 928 K as shown in (c).
BMG (3.0 mm in diameter), and (Fe0:72 B0:24 Nb0:04 )95:5 Y4:5
BMG which is once heated up to 928 K to induce the phase
transformation. In order to show the effect of Y addition,
DSC curve of the ternary Fe72 B24 Nb4 was also shown in
Fig. 2. The exothermic peak at around 928 K of the as-cast
(Fe0:72 B0:24 Nb0:04 )95:5 Y4:5 BMG is so unique that there have
been no reports on this phenomenon. The exothermic peak
was apparently observed on DSC trace of the alloys with
> 3:5 at% Y elements. If a glass transforms to a more stable
phase with a lower free energy level like crystal, the surplus
energy could be released from the internal to external
systems. From this point of view, it can be expected that the
present exothermic reaction results from crystallization,
phase separation or chemical short-range ordering. To
investigate the exothermic transformation on the DSC
traces, the specimen was heated up to 928 K, i.e. the top of
exothermic peak at a heating rate of 0.67 K/s by DSC,
then cooled down to the room temperature. Resulted from
this heat treatment, the exothermic peak was clearly
disappeared without shifting Tg and Tx , on the DSC trace
(c) of the annealed sample in Fig. 2. TEM observation on
the (Fe0:72 B0:24 Nb0:04 )95:5 Y4:5 BMGs annealed at 922 K
(¼ Tg þ 51 K) for 30 s was carried out, and the results are
shown in Fig. 3 as well as that of the as-cast state for
comparison. No apparent evidence for the phase transformation was observed in the bright field images, diffraction
patterns and HRTEM of both the as-cast and annealed
specimens. This indicates that (nano-)crystallization could be
excluded from the causes of the present exothermic reaction.
Thus the phase separation or chemical short-range ordering is
remained as a candidate to explain this phenomenon. In this
case, the apparent exothermic phenomenon was observed in
DSC curves and the most considerable origin of the
phenomenon is chemical short-range ordering. The phase
separation is also expected to occur due to the positive
enthalpy between Nb and Y, but the contents of both Nb
(4 at%) and Y (4.5 at%) are too small to bring about any
phase separations. Furthermore, there did not appear any
evidences of phase separation even though HRTEM observations as seen in Fig. 3.
3.3
Thermal stability and nanocrystallization of
(Fe0:72 B0:24 Nb0:04 )95:5 Y4:5 BMG
(Fe0:72 B0:24 Nb0:04 )95:5 Y4:5 glassy ribbon was annealed at
Tg (¼ 871 K) for 5.4 ks and 21.6 ks to investigate thermal
stability of the alloy. There are no crystallization peaks
observed in the XRD patterns for these samples as shown in
Fig. 4(a) although Tx (thus Tx ) in the DSC curves changed
with the annealing time, on the other hand Tg kept a constant
value as shown in Fig. 4(b). It is considered to be the first
report on the excellent thermal stability at Tg , that thermal
stability of over than 21.6 ks at Tg , especially in Fe-/Co-
(a)
(b)
(c)
(d)
Fig. 3 TEM micrographs, the corresponding selected area diffraction (SAD) patterns and HRTEM images of as-quenched and annealed (at 922 K for 30 s) of
(Fe0:72 B0:24 Nb0:04 )95:5 Y4:5 BMGs ((a) and (b) are of the as-quenched, and (c) and (d) are of the as-annealed samples).
Excellent Thermal Stability and Bulk Glass Forming Ability of Fe-B-Nb-Y Soft Magnetic Metallic Glass
Cu-K α
Intensity (arb.unit)
(Fe0.72B0.24Nb0.04 )95.5Y4.5
Heating Rate = 0.67K/s
(Fe 0.72B0.24Nb0.04)95.5Y4.5
871K × 5.4 ks
871K × 21.6 ks
Exothermic
871K (Tg ) × 21.6 ks
871K (Tg ) × 5.4 ks
509
as-Q
∆ Tx
Tg
Tx
as-Q
20
30
40
50
60
70
80
800
2 θ / degree
900
1000
Temperature/K
(b)
(a)
Fig. 4 (a) XRD and (b) DSC traces obtained from (Fe0:72 B0:24 Nb0:04 )95:5 Y4:5 glassy ribbon at as-quenched and annealed (@ 871 K (¼ Tg )
for 5.4 ks and 21.6 ks) states.
Cu-K α
Fe23B6
Intensity (arb.unit)
(Fe 0.72B0.24Nb0.04 )95.5Y4.5
922K × 3.6 ks
922K × 1.2 ks
922K × 600 s
922K × 300 s
922K × 30 s
20
30
40
50
60
70
80
2 θ / degree
Fig. 5 XRD patterns obtained from (Fe0:72 B0:24 Nb0:04 )95:5 Y4:5 glassy
ribbon annealed at 922 K (¼ Tg þ 51 K) with various annealing time as
for 30 s, 300 s, 600 s, 1.2 ks and 3.6 ks. The primary phase of this BMG was
found to be Fe23 B6 .
based BMGs. The sustaining of the glassy phase during the
annealing for 21.6 ks at Tg is a promising characteristic for
perpendicular magnetic memory device technology as soft
magnetic underlayer and also ultra-small structural devices,
since this Fe-based BMGs can be manufactured delicately
with using the Newtonian viscous flow at around Tg such by
die-casting, and micro-/nano-scale imprinting techniques.
(Fe0:72 B0:24 Nb0:04 )95:5 Y4:5 glassy ribbon was annealed at
922 K (¼ Tg þ 51 K) with various annealing time as for 30 s,
300 s, 600 s, 1.2 ks and 3.6 ks to induce devitrification. The
XRD patterns of the annealed specimens in Fig. 5 show
crystallization behavior of the glassy ribbon. The sharp peaks
diffracted from crystalline phase(s) were appeared at around
43 degree from the annealing time of 600 s, 1.2 ks and 3.6 ks.
Even after the long time annealing for 3.6 ks, a broad halo
pattern was still observed clearly from 35 to 50 degree. This
means that glassy and crystalline phases coexist simultaneously after the annealing, and each of the phases has the
high thermal stability as mentioned above even in the
supercooled liquid region. The crystalline phase was identified to Fe23 B6 , which is observed well in Fe-based
amorphous ribbons and BMGs containing B and Nb
elements.12) For more detail observation on the phase
transition from the glassy to crystalline phases, the glassy
ribbons annealed at 922 K for 1.2 ks was observed by TEM.
The electron diffraction pattern, dark field (DF) and HRTEM
images of the one annealed at 922 K for 1.2 ks were revealed
in Figs. 6(a), (b) and (c)–(f), respectively. As shown in
Fig. 5, the samples annealed for 1.2 ks at 922 K consist of
glassy and nano-crystalline phases. The size of nanocrystal of
the samples annealed for 1.2 ks seems about 50 nm from the
DF image in Figs. 6(a). The volume fraction of nanocrystalline phase against the glassy matrix can be estimated to be
about 5% by calculating the area of nano-crystalline particle
in the images. From Figs. 6(c) and (d) a nanocrystalline
aggregation of which size is about 50 nm is observed, and is
found to consist of 10 nm size nanocrystalline particles. As
shown in Figs. 6(e) and (f) the nanocrystalline particles are
showing the ordered configuration, but the interfaces between nanocrystalline phase and glassy matrix are not
facetted. The lattice parameter of the nanocrystalline phase
is deduced to be about 1 nm, thus this phase is identified to be
the Fe23 B6 metastable phase with using the XRD results in
Fig. 5. This metastable phase has been often observed in Febased bulk glass formers. In the superccoled liquid of these
alloys, Fe23 B6 type clusters are considered to be formed and
to suppress the atomic diffusion required for formation of the
stable crystalline phase, i.e. -Fe. This may help the glass
formation in these alloys.
3.4
Soft magnetic properties of (Fe0:72 B0:24 Nb0:04 )95:5 Y4:5
BMG and their improvement by heat treatment
Figure 7 shows outer morphology and surface appearance
of a cast (Fe0:72 B0:24 Nb0:04 )95:5 Y4:5 rectangular glassy rod
with a length of 12 mm, a height of 2 mm and width 2 mm.
Saturation magnetization was measured by VSM with using
the cast rectangular glassy rod owing to reduce errors from
fluctuation of density. In the case of amorphous ribbons, it is
so difficult to measure density correctly due to small weight
of sample. On the other hands, bulk sample enables us to
achieve relatively correct value of density. Density was
calculated by Archimedes method with using a tetrabromoethane liquid. There were not appeared any differences of
510
S. Lee et al.
Fig. 7 Outer morphology and surface appearance of a cast rectangular
(Fe0:72 B0:24 Nb0:04 )95:5 Y4:5 glassy alloy rod with a length of 12 mm, a
height of 2 mm and width 2 mm.
1.0
Magnetization, J (T)
Rectangular bulk rod
Fig. 6 (a) Dark Field (DF) image, (b) selected diffraction pattern and (c–f)
HRTEM images obtained from the (Fe0:72 B0:24 Nb0:04 )95:5 Y4:5 melt spun
ribbon annealed for 1.2 ks at 922 K (¼ Tg þ 51 K).
0.5
12mm × 2mm × 2mm
0.0
-0.5
-1.0
6
-1.0x10
5
0.0
-5.0x10
5
6
1.0x10
5.0x10
Applied Magnetic Field, H/Am-1
Fig. 8 Magnetization curve of a cast rectangular (Fe0:72 B0:24 Nb0:04 )95:5 Y4:5
glassy alloy rod with a length of 12 mm, a height of 2 mm and width 2 mm.
density between as-cast and annealed samples in this study.
As seen in Fig. 8, magnetization curve shows that the
annealed rectangular glassy rod has a saturation magnetization, Js of 0.8 T, which is approximately as same as that for
as-cast one. The reason why the saturation magnetization is
not changed with thermal treatment is caused by the constant
density before/after annealing. The saturation magnetization,
Js of 0.8 T is lower than that for conventional Fe-based
amorphous alloys.19–21) Compared to the references of Febased amorphous alloys including Nb or Y as shown in
Table 2, Js of the composition is lower than that of Fe-based
alloys including Nb or Y. A cast ring with a thickness
0.5 mm, an outer diameter of 10 mm and an inner diameter
of 6 mm was prepared to investigate coercivity, Hc as shown
in Fig. 9. The coercivities were measured by B-H analyzer
with using both an amorphous ribbon and a cast
(Fe0:72 B0:24 Nb0:04 )95:5 Y4:5 glassy ring in order to investigate
the effect of shape on coercivity. Figure 10 shows the B-H
hysteresis curves of a cast (Fe0:72 B0:24 Nb0:04 )95:5 Y4:5 glassy
ring and an amorphous ribbon after annealing at 821 K
(¼ Tg 50 K) for 300 s. The annealed cast ring has Hc of
0.8 A/m, which is descended from 1.0 A/m for that of as-cast
Table 2 A comparison of saturation magnetization (Js ) and coercivity (Hc ) for various Fe-based amorphous alloys.
Composition
Js (T)
Hc (A/m)
References
(Fe0:75 B0:15 Si0:10 )99 Nb1
—
1.50
3.7
19)
(Fe0:75 B0:15 Si0:10 )96 Nb4
—
1.47
2.9
19)
Y4 Fe76 B20
—
1.56
—
20)
Y6 Fe72 B22
—
1.47
4.0
20)
Fe80 B11 Si9
(Fe0:72 B0:24 Nb0:04 )95:5 Y4:5
—
Ribbon (as-quenched)
1.59
—
—
20
21)
This study
(Fe0:72 B0:24 Nb0:04 )95:5 Y4:5
Ribbon (annealed)
—
0.2–0.5
This study
(Fe0:72 B0:24 Nb0:04 )95:5 Y4:5
Bulk (as-quenched)
0.8
1.0
This study
(Fe0:72 B0:24 Nb0:04 )95:5 Y4:5
Bulk (annealed)
0.8
0.8
This study
Excellent Thermal Stability and Bulk Glass Forming Ability of Fe-B-Nb-Y Soft Magnetic Metallic Glass
511
bulk glassy samples which can have some defects due to the
wrong casting condition. In the case of other amorphous
ribbons, it has been reported that densities may increase by
appropriate thermal treatments due to decrease of vacancy in
the ribbons.24) On the other hands, vacancy in a bulk sample
is expected to remain in spite of thermal annealing. Therefore, the best casting condition should be acquired to solve
the original defects problem during casting process for
applications of the Fe-based BMGs.
4.
Fig. 9 Outer morphology and surface appearance of a cast
(Fe0:72 B0:24 Nb0:04 )95:5 Y4:5 glassy alloy ring with a thickness 0.5 mm, an
outer diameter of 10 mm and an inner diameter of 6 mm.
ring
Magnetization, J (T)
0.04
0.02
ribbon
0.00
-0.02
-0.04
-12 -10 -8 -6 -4 -2
0
2
4
6
8
10 12
Applied Magnetic Field, H/Am-1
Fig. 10 B-H hysteresis curves of a cast (Fe0:72 B0:24 Nb0:04 )95:5 Y4:5 glassy
ring and an amorphous ribbon after annealing at 821 K (¼ Tg 50 K) for
300 s.
one. The annealed amorphous ribbon has Hc of 0.2–0.5 A/m,
which also decreased from 20–30 A/m for that of asquenched one.
The Hc of 0.2–0.5 A/m after annealing is the lowest value
among the Fe-based soft magnetisms which have been
developed up to date. The origin of the novel Hc of 0.2–
0.5 A/m may be caused by high structural homogeneity in the
Y-added alloy closely related to the high GFA and oxygen
scavenging effect through the formation of yttrium oxide
during arc-melting and copper mold casting/melt-spinning
processes.22) Y atoms have a stronger affinity (Y2 O3 ¼
1904 KJ/mol) to the oxygen atoms than Fe atoms
(Fe2 O3 ¼ 820:5 KJ/mol),23) Y addition thus can decrease
the coercivity by descend of ferromagnetic portion from the
alloy.
The difference of Hc between 0.8 A/m of a glassy ring and
0.2–0.5 A/m of an amorphous ribbon can be explained by the
defects in the bulk glassy ring sample. The possibility of
defects such as contamination and vacancy might be higher
in bulk glassy samples than in amorphous ribbons. In this
study, a large of fluctuation on density was discovered in the
Conclusion
The effects of Y addition on the glass forming ability
(GFA) of a Fe-B-Nb marginal glass former, and annealing
effects on glassy/supercooled liquid phases and soft magnetic properties in the multicomponent Fe-B-Nb-Y alloy
system were investigated. The results can be summarized as
follows.
(1) In a Fe-B-Nb ternary marginal glass forming system, a
Fe72 B24 Nb4 (at%) alloy was found to have the largest
supercooled liquid temperature region (Tx ) of 39 K at
a heating rate of 0.67 K/s and critical glass forming size
of 0.02 mm.
(2) Y addition was confirmed to improve GFA of the
Fe72 B24 Nb4 alloy. The maximum critical glass forming
diameter in (Fe0:72 B0:24 Nb0:04 )100x Yx (x ¼ 0 6) system was 7 mm (350 times larger than the Fe72 B24 Nb4
base alloy) of an (Fe0:72 B0:24 Nb0:04 )95:5 Y4:5 alloy.
(3) Characteristic exothermic phase transformation within
the supercooled liquid region was observed for the
alloys with 3:5 at% Y element. This is considered to
be resulted from a chemical short range ordering
associated with the large positive heat of mixing
between Nb-Y elements.
(4) (Fe0:72 B0:24 Nb0:04 )95:5 Y4:5 alloy was found to maintain a
single glassy phase even after a heat treatment at Tg
(¼ 871 K) for 21.6 ks (¼ 6 hours). This indicates that
this bulk metallic glass has distinct higher-thermal
stability with compared to other Fe-/Co- based bulk
metallic glasses developed up to date.
(5) Saturation magnetization and coercivity of the as-cast
(Fe0:72 B0:24 Nb0:04 )95:5 Y4:5 glassy ring were found to be
0.8 T and 0.8 A/m after annealing, respectively. In
addition, lower coercivity of 0.2–0.5 A/m was achieved
by the appropriate heat treatment in the amorphous
ribbon sample. The origin of excellent soft magnetism
is considered to be caused by homogeneity and oxygen
scavenging effect.
Acknowledgement
This work was supported by a Grant-in-Aid for Scientific
Research on Priority Areas, ‘‘Materials Science of Bulk
Metallic Glasses’’, from the Ministry of Education, Science,
Sports and Culture of Japan.
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